1. Field of the Invention
This invention relates to masterbatches and their method of use for producing polyolefin articles, particularly films. High density polyethylene can be incorporated into the masterbatch alone or in combination with a hydrocarbon resin and/or a polyolefin and when used in combination with a hydrocarbon resin causes the masterbatch to solidify more rapidly and be pelletized more efficiently (improved compounding efficiency). These masterbatches ultimately result in extruded polyolefin articles, such as cast polypropylene films, of optimal stiffness and ductility.
2. Discussion of Background
Polyolefins are plastic materials useful for making a wide variety of valued products due to their combination of stiffness, ductility, barrier properties, temperature resistance, optical properties, availability, and low cost. Being a semi-crystalline polymer, a number of these important properties such as stiffness, barrier properties, temperature resistance, and optical properties, depend on the ability of the polyolefin to crystallize in the most effective manner, and to the desired degree.
The process for forming a polyolefin product strongly affects the crystallization behavior of the material and its ultimate properties. For instance, when polypropylene is cast into thin film, the polymer cools so quickly that the ultimate level of crystallinity is reduced by this "quenching" process, and correspondingly the stiffness of the film is reduced. Cast polypropylene films typically exhibit a stiffness, measured as tensile modulus, of nominally 100 Kpsi. Highly oriented polypropylene (OPP) films typically-exhibit modulus values 2-4 times higher than the values for cast polypropylene film while non-oriented thick molded articles typically exhibit modulus values nominally 50% to 100% higher then cast polypropylene film. Also when making cast film, it is important that the polypropylene melt solidify quickly to promote high production rates, and also that the crystalline regions which are formed are not so large in size that they confer haze to the film.
Other molded polyolefin articles, particularly thin gauge products made by thermoforming, injection molding, or blow molding, are subject to similar constraints. Faster crystallization which permits rapid demolding and stiffer products is desired, as well as good optical properties promoted by small crystalline domain size.
As a means for improving the stiffness of polyolefins, the addition of a high softening point hydrocarbon resin to polyolefins, such as polypropylene, is known. The composition of the hydrocarbon resin must be such that it exhibits a significantly higher glass transition temperature (Tg) than the amorphous regions of the polypropylene (Tg around -10.degree. C.), and the hydrocarbon resin must be highly compatible in the polypropylene. It is believed that the effect of the hydrocarbon resin is to increase the Tg of the amorphous polypropylene fraction and by doing so increase its tensile modulus at temperatures below 38.degree. C.
The hydrocarbon resins described above are friable solids which exhibit very low melt viscosity at the temperatures normally used to process polyolefin. An effective way to blend hydrocarbon resin into polyolefin is in a separate compounding step prior to the final use of the blend. It is difficult to incorporate hydrocarbon resin into polypropylene during an actual conversion step (for example film casting, sheet extrusion, etc.) because of the hydrocarbon resins dusting characteristics and low melt viscosity. A more effective way to incorporate hydrocarbon resin into polyolefin during the conversion step is to add the resin in concentrate form as a mixture of resin with polyolefin. U.S. Pat. No. 5,213,744 describes a process of forming a concentrate consisting of a simple binary mixture of hydrocarbon resin and polyolefin, and using this concentrate as a more effective way of incorporating hydrocarbon resin into a polyolefin film formulation at a level of 5 wt. % to 30 wt. %.
Although the stiffening caused by adding hydrocarbon resin is desirable, it can be achieved only by adding high levels of hydrocarbon resin (typically at or above 5 wt. %) to the total polyolefin formulation, and only if the softening point of the hydrocarbon resin is 100.degree. C. or higher, the stiffening effect increases as the hydrocarbon resin content and softening point increase. While the stiffening effect caused by the addition of hydrocarbon resin to polyolefins, is desirable, adding high levels of resin (e.g., above 5 wt. %), has a negative impact on ductility and results in increased formulation cost. Therefore, it would be highly desirable to enhance polyolefin stiffness by addition of hydrocarbon resin to the polypropylene at levels below 5 wt. % and preferably below 3 wt. %.